The compression ratio of an internal combustion engine is defined as the ratio of the cylinder volume when the piston is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine. In conventional engines, the compression ratio is fixed and thus the engine efficiency cannot be optimized during operating conditions to improve fuel economy and engine power performance.
In variable compression ratio (VCR) engines, the engine may be equipped with various mechanisms to mechanically alter the volumetric ratio between the piston TDC and BDC, allowing for the compression ratio be varied as engine operating conditions change. As a non-limiting example, the VCR engine may be configured with a piston displacement changing mechanism (e.g., an eccentric) that moves the pistons closer to or further from the cylinder head, thereby changing the size of the combustion chambers. Still other engines may alter a cylinder head volume.
One example approach for leveraging the benefits of a VCR mechanism is shown by Kolmanovsky et al in U.S. Pat. No. 6,553,949. Therein, a higher compression ratio may be used in low engine speed-load regions to increase thermal efficiency. A lower compression ratio may be used in higher engine speed-load regions. Further, knock may be mitigated in the low speed-load region using spark retard, while knock is mitigated in the higher speed-load region via adjustments to the compression ratio. When the engine becomes spark limited while operating in the higher compression ratio, the combustion phasing retard may erode the thermal efficiency benefit of the compression ratio. At that time, the compression ratio may be lowered while spark timing is advanced so as to provide a more efficient balance between combustion phasing and thermal efficiency. In addition, hunting between different levels of compression is reduced and a given compression ratio may be used more effectively.
However the inventors herein have identified potential issues with such a system. As one example, the VCR mechanism may degrade resulting in the engine operating at a compression ratio different from that desired. For example, the VCR mechanism may get stuck. If the mechanism gets stuck in a higher than desired compression ratio, such as when the mechanism gets stuck while in a higher compression ratio setting, or during a transition to a lower compression ratio setting (but before reaching the lower compression ratio setting), the risk of engine knock and pre-ignition may increase. In particular, higher pressures at the time of spark can lead to higher temperatures and higher probability of auto-ignition of the unburned end gases. Knock and pre-ignition incidence can reduce the life of engine components.
In one example, the above issue may be at least partly addressed by a method for an engine comprising: mechanically varying a compression ratio of an engine via a variable compression ratio mechanism; and responsive to the mechanism being degraded, limiting an engine load. In this way, knock or pre-ignition induced due to VCR mechanism degradation is reduced.
As an example, an engine may be configured with a VCR mechanism that, when actuated, mechanically varies the position of a piston within the combustion chamber, thereby varying the compression ratio. Responsive to engine operating conditions, the compression ratio may be varied, such as by applying a relatively higher compression ratio at lower engine loads to leverage an increased thermal efficiency benefit, while transitioning to a relatively lower compression ratio at higher engine loads to leverage a combustion phasing benefit. The VCR mechanism may become stuck during the transition between the higher and lower compression ratio settings. Degradation of the VCR mechanism may be inferred based on feedback regarding a position of the VCR mechanism, such as via a position sensor coupled to the mechanism. For example, if the detected position is different from the commanded position, it may be inferred that the mechanism is stuck in the wrong compression ratio. Alternatively, degradation of the VCR mechanism may be inferred responsive to a higher than expected knock incidence and/or adaptive knock being retarded to a threshold earlier than expected. The stuck compression ratio may be determined based on position sensor feedback or based on the knock incidence. The engine load may then be limited to a clip that corresponds to the highest load possible for the given stuck compression ratio. An intake air charge may be reduced to limit the load, such as by reducing the opening of an intake throttle or by increasing the opening of an exhaust waste-gate valve. In addition, borderline spark may be adjusted based on the stuck compression ratio. Spark timing may then be advanced.
In this way, a variable compression ratio mechanism may be better protected from recurrent knock and pre-ignition. The technical effect of limiting intake aircharge to limit engine load responsive to degradation of the variable compression ratio mechanism is that recurrent knock may be averted. In particular, thermal stress that may be incurred in a cylinder due to operation at a higher than intended compression ratio may be reduced (such as may occur due to the mechanism being stuck in a higher than intended compression ratio setting). In addition, the engine may be operated with spark advanced from a spark retard limit. Overall, engine component life may be extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.